The present invention is directed toward cable supported bridges, and more particularly toward a method of modifying the vertical profile of an existing cable supported bridge structure.
There are two primary types of cable supported bridge, the suspension bridge, and the cable stayed bridge. The superstructure of a suspension bridge consists of stabilizing trusses and the bridge roadbed. The superstructure is suspended from vertical hanger cables which extend upward to catenary cables. The catenary cables span the distance between adjacent suspension bridge towers and ride freely across the towers, transmitting load to anchorages at either end of the bridge. At the completion of the construction of a suspension bridge, dead load stresses in the superstructure members are very small, while dead load stresses in the hanger cables, catenary cables, anchorages and towers are quite large.
A typical cable stayed bridge also has one or more load bearing towers rising above the bridge superstructure. In contrast to the suspension bridge however, stay cables run from the tower diagonally to attachment points on the bridge superstructure.
Cable supported bridges are highly non-linear structures. Large displacements associated with construction, wind, seismic, dead and live loads are accommodated by the cables and other flexible bridge components and systems. Thus, any change in the load applied to the bridge superstructure can result in significant and unproportional change or displacement in the vertical profile of the bridge superstructure.
Most major cable supported bridges of the world span active shipping channels. Accordingly, maintenance of the clearance between the lowest point of the bridge superstructure and the water level is critical to assure unimpeded traffic under the bridge. Unfortunately, the flexibility inherent in a typical cable supported bridge structure allows significant downward deflection of the bridge superstructure if additional dead load is added. For example, the addition of a bike path or additional lanes of automobile traffic to an existing bridge superstructure can cause downward deflection of the superstructure which is substantial enough to obstruct the shipping lanes under the bridge.
Historically, when faced with this problem, bridge engineers have reduced the weight of the original bridge superstructure to offset the additional weight of the new load. For example, the weight of the addition of a bicycle path to an existing bridge superstructure might be offset by replacing the existing original concrete traffic deck with an orthotropic steel bridge deck. Upon completion of all phases of such a project, the overall weight of the bridge superstructure would remain unchanged, and the vertical profile of the bridge superstructure would remain substantially unchanged. The replacement of a concrete traffic deck with an orthotropic steal bridge deck requires periodic bridge lane closures over the course of a lengthy construction period. In addition, the capital cost of bridge deck replacement is very high.
The present invention is directed toward overcoming one or more of the problems discussed above.
The present invention is a method of modifying the vertical profile of a cable supported bridge. Alternatively, the invention is a cable supported bridge modified by the method disclosed herein. The modification of the vertical profile of a bridge can increase the clearance between the bottom of the bridge superstructure and the water or ground level below. In the alternative, the method of this invention can be used to preliminarily modify the vertical profile of a cable supported bridge allowing for subsequent increase in the dead load supported by the bridge without causing permanent alteration of the vertical profile of the bridge superstructure.
The method is applicable to a cable supported bridge having one or more towers supporting one or more cables attached to a bridge superstructure. The method is applicable but not limited to bridges built in either a cable stayed or suspension bridge configuration.
The method consists of sequentially adjusting the lower end of a series of supporting cables downward relative to the bridge superstructure or, in the case of a cable stayed bridge, adjusting the lower end of the series of cables away from the support tower, and sequentially adjusting an attachment structure associated with the lower end of each cable to maintain the distance each cable has been adjusted. The above actions effectively shorten each of a series of cables. The adjustments preferably proceed according to a pre-specified plan, known as an adjustment sequence. The effect upon the vertical profile of the bridge from the adjustment of any given cable is quite small; however, the net result of the adjustment of a series of cables is the modification of the vertical profile of the bridge superstructure. Additionally, after the vertical profile of the bridge has been modified, additional dead load can be added to the superstructure. By proper implementation of the method of this invention, the downward deflection resulting from the addition of new dead load to the bridge superstructure will offset the initial modification of the bridge""s vertical profile resulting in clearance between the bottom of the bridge superstructure and the water line which is no less than that originally maintained.